Relapsing fever is a vector-borne disease long associated with poverty and human suffering in areas of social disruption across Africa and Asia. In the western U.S., relapsing fever is strictly tick-borne with a highly biocomplex ecology consisting of a number of Borrelia species and associated soft tick vectors (in the genus Ornithodoros), and numerous mammalian hosts. In addition to ecological complexity, a hallmark of relapsing fever systems is its genetic complexity. Once in the host, the relapsing fever spirochete avoids host immunity and hence natural selection via hypervariability in major surface antigens, namely expressed vmp (vsp and vlp) genes. The bacterium utilizes a reserve of pseudogenes located on multiple plasmids that allow recombinant variants to be expressed during successive waves of spirochetemia. Additionally, relapsing fever spirochetes use factor-H binding protein (the fhbA gene located on plasmid, lp200) to avoid host complement. Interaction between pathogen and host immunity appears to create conditions where plasmid-containing strains are retained and those without are removed. The role of plasmid evolution and the degree to which these relapsing fever genotypes are maintained in distinctive host tropisms has not yet been elucidated. Evolution of relapsing fever spirochetes is hypothesized to occur through direct host-pathogen interactions at the vmp and fhbA locus through diversifying selection to maintain variants for diverse host species. The evolutionary significance of separate circulating strains is that they may reflect incipient speciation events, and due to the direct interaction of host immunity with the surface proteins of relapsing fever agents it follows that variation in host diversity will influence the diversity and maintenance of relapsing fever spirochetes in a given area. We hypothesize that multiple genotypically distinct strains of Borrelia hermsii have co-evolved with endemic rodent reservoir populations and that bacterial genotypes will reflect the clonal nature of these relapsing fever populations within their rodent reservoirs. We predict B. hermsii genotypes in a given area will vary depending on the amount of host population structure encountered and that the interaction of diverse strains within a population can modify the evolutionary trajectory of a specific bacterial population. As short-term goals for this discovery- driven research, we propose to complete the following specific aims: 1) Isolate and genotype relapsing fever strains from wild rodents at six established sites endemic for relapsing fever in California and Nevada, 2) determine the population structure of wild rodent reservoirs using microsatellite analysis, and 3) analyze the degree to which host population structure correlates with diversity in relapsing fever genotypes encountered at each endemic location. The long-term goals of our research are to discover the mechanisms by which relapsing fever spirochetes are maintained in nature, how diversification and speciation in pathogens contribute to the emergence of novel strains, and how both patterns relate to infection risk for humans.